feat: deduct vendor-country bank holidays from billable days

[Sajjon]

Summary

Adds an opt-in off_on_bank_holidays flag to invoice ServiceFees. When enabled, public holidays in the vendor's country are deducted from billable working days for day- and **hour-**granularity rates (monthly/fortnightly fixed rates are unaffected — a fixed monthly fee doesn't shrink for a holiday).

Closes the feature request to be "off on bank holidays".

Feasibility outcome

• API: Nager.Date — free, no key, no rate limit, 195 countries incl. 🇸🇪 SE / 🇺🇸 US / 🇬🇧 GB (verified against the live AvailableCountries endpoint). Sweden is fully covered by the global API; the Swedish-specific dryg.net is not needed.
• Chosen over the offline py-holidays-rs crate (beta, 1★ — supply-chain risk).

Design (mirrors existing patterns)

• Online fetch + RON disk-cache fallback, modeled 1:1 on the existing exchange-rate fetcher. Cache at ~/.klirr/cached_holidays.ron, keyed ISO country → year; repeat runs work offline.
• Name → ISO mapping: the vendor's free-text country string is resolved via a curated alias table (Sweden/Sverige→SE, England/UK→GB, USA→US, ~45 countries).
• Graceful degradation: an unmappable country or a failed lookup logs a warning and skips the deduction — it never blocks PDF generation.
• Backward compatible: flag defaults to false and deserializes from legacy RON via #[serde(default)].

Changes by crate

• foundation: CountryCode newtype + resolver; BankHolidays newtype; BankHolidaysFetcher behind a new bank-holidays feature; working_days_between/quantity_in_period now skip holiday dates (holidays on weekends are no-ops).
• core-invoice: ServiceFees.off_on_bank_holidays; resolve_bank_holidays wired into prepare_invoice_input_data.
• cli: yes/no prompt in the service-fees wizard.

Tests

• Country resolver (Sweden/UK/US aliases, unknown → None).
• Holiday fetch with mocked responses (Public-type filter, cache hit/miss, gibberish-cache reset) — no network in tests.
• working_days_between deducting a known holiday; holiday-on-weekend no-op; hours = days×8 deduction.
• ServiceFees legacy-RON deserialization (no field → false).
• Graceful-degrade path (enabled flag + unmappable country → empty, no network).

All unit + doc tests pass; clippy clean for new code.

Note on CI/fixtures: the render-typst image-fixture tests are environment-sensitive (imagemagick) and fail identically on a clean main on the dev machine (same similarity value), so they're unrelated to this change — samples use the default flag=false and render identically.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 81.11111% with 34 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.96%. Comparing base (b9c0ca6) to head (10bbe98).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/foundation/src/bank_holidays.rs	63.76%	25 Missing ⚠️
...re-invoice/src/logic/prepare_data/bank_holidays.rs	50.00%	7 Missing ⚠️
...-invoice/src/models/data/submodels/service_fees.rs	0.00%	1 Missing ⚠️
crates/foundation/src/models/country_code.rs	98.46%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #47      +/-   ##
==========================================
- Coverage   97.07%   95.96%   -1.11%     
==========================================
  Files         104      108       +4     
  Lines        2287     2452     +165     
==========================================
+ Hits         2220     2353     +133     
- Misses         67       99      +32     

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